A STUDY ON THE REDUCTION OF CHIP EVACUATION TORQUE IN ULTRASONIC ASSISTED DRILLING OF SMALL AND DEEP HOLES

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 6, June 2018, pp , Article ID: IJMET_09_06_101 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed A STUDY ON THE REDUCTION OF CHIP EVACUATION TORQUE IN ULTRASONIC ASSISTED DRILLING OF SMALL AND DEEP HOLES Van-Du Nguyen Faculty of Mechanical Engineering, Thai Nguyen University of Technology, Viet Nam Ngoc-Hung Chu Faculty of International Training, Thai Nguyen University of Technology, Viet Nam ABSTRACT This paper presents an experimental study to examine the effectiveness of ultrasonic assisted drilling (). The cutting torque has been typically considered as independent the hole depth. It has been found that the cutting torque in drilling can be reduced by 25% by applying ultrasonic assistance. As deeper depths, a new component named as chip evacuation torque is generated, making the total torque exponentially increase. The substantial increase of the total torque can lead to a risk of drilling breakage. A critical depth is defined as the depth where the total torque is still lower than the allowance torque, which was determined the broken condition of the drill bits. Using response surface technique in design of experiments, this study found that can increase the critical depth in small and deep drilling by 1.4 times compared to conventional drilling, with a safety factor of 5. This helpfulness of can be further developed to combine with other common techniques such as peck drilling or grooving tool for deep and small drilling. Keywords: Ultrasonic Assisted Drilling, Chip evacuation, Deep drilling, Aluminum Alloy, Depth hole. Cite this Article: Van-Du Nguyen and Ngoc-Hung Chu, A Study on the Reduction of Chip Evacuation Torque in Ultrasonic Assisted Drilling Of Small and Deep Holes, International Journal of Mechanical Engineering and Technology, 9(6), 2018, pp INTRODUCTION Drilling is one of the major mechanical machining processes, as about one-third of all manufacturing operations [1]. A drilling process is seemed to be very simple with only two required motions of cutting and feeding. However, drilling is a complex operation: the cutting editor@iaeme.com

2 A Study on the Reduction of Chip Evacuation Torque in Ultrasonic Assisted Drilling Of Small and Deep Holes edge is composed of a chisel edge and a cutting lip, the cutting speed and the tool geometry vary along the cutting edge [2]. Different other conventional machining processes such as turning and milling where chips are free of external forces after leaving the cutting area, the chips in drilling are constrained by the drill flute, causing the change of chip shape and thus the drilling forces and torque [3]. In drilling, long stringy chips resulting ductile plastic deformation of the workpiece materials cause severe chip clogging within the narrow helical flutes and edge chipping. Chip clogging would lead to rapid tool wear and breakage, poor tool life, poor hole quality and the productivity. Cutting fluid, if applied, could not efficiently reach the cutting zone [4]. Dry machining apparently has severe advantage of reducing the production costs, the environment risks, the health risks, and the problems involved with recycling chips [5]. Aluminum alloys have been widely used in applications where materials with high strength-to-weight ratio are required, such as in the rapid growing automobile and aerospace industries. Generally, aluminum alloys have been considered as one of the easiest machining materials. However, these materials are considered as the most critical materials for dry drilling [6, 7]. In addition, emerging trend of dry cutting, an environmentally friendly machining technique, has also given a real challenge in drilling aluminum alloys [8]. During the dry drilling process of aluminum alloys, long and ductile chips, especially in deep holes, tend to bend and coil and thus cause packing of the drill flutes, interferes with chip ejection [9]. High tendency of aluminum to adhere to cutting tools will cause persistent elongated contact with cutting edges and flutes and thus produce a high risk of tool breakage. In deep hole drilling, where the aspect ratio increase further to more than about three to four times (L/D 3-4) of twist drills [9], the torque increase exponentially [2], [3], [10], [11], [12], [13], whereas the thrust (axial) force remains relatively constant [12], [13] or increase almost linearly [2]. The most common assumptions for such trends have been the friction among the tool flutes, the chips and the hole walls [10], [2], [3], [14], [15]. As the drilling depth increases, an increased amount of chips would fill up the flutes, leading to chipclogging, i.e. the jamming of chips inside the flute. Chip evacuation torque (thrust) is defined as the increase in the torque (thrust) after full lip engagement, till the critical depth is reached. Each of the (thrust) force and the torque thus will be considered as a combination of two components: the cutting force/torque and the chip-evacuation force/torque. The cutting force and cutting torque are the corresponding values obtained when drill lips are fully engaged into the workpiece. The chip-evacuation force and torque were then calculated by subtracting the cutting force and cutting torque the total observed data [10]. The addition of chip evacuation torque results in excessive torsional stress, and once the maximum torsional strength of the drill are reached, drill breakage would occur [10], [16], [13]. To improve chip evacuation, several attempts have been made, such as using groove-type chip breaker [15], peck drilling [16] or using different flute geometries [11]. Recently, a new promising technique, well known as ultrasonic assisted drilling (), has been investigated in drilling. Compared conventional drilling (CD), there have been shown significant reduction of thrust force [17-21], improvements in built-up edge [17, 22], burr size [17, 23], tool life [19] and hole oversize [17] which can be taken ultrasonic assisted drilling (). A significant reduction of drilling torque of 50% when applying for aluminum has been found in the study of Neugebauer and Stoll [24]. For other materials, Makhdum et al. [25] found that there was a significant reduction in drilling torque of 80% when applying to dry carbon-fiber-reinforced plastics. Ostasevicius et al. showed that applying for XC48 steel [26] can reduce drilling torque by 13 to 20% compared to CD. Alam et al. found a torque reduction of around 14% when applying for bone [27]. However, in all mentioned studies, drilling tests have been implemented for shallow holes, where the aspect ratio L/D editor@iaeme.com

3 Van-Du Nguyen and Ngoc-Hung Chu was only ranged 2 to 4, while in many applications the depth of required holes are ten or more times of hole diameter [28]. Besides, effectiveness of in terms of chip evacuation torque has rarely been found yet. This paper presents an experimental study to examine the effectiveness of ultrasonic assisted drilling (). The advantage of in reducing the cutting torque is validated in order to highlight the effectiveness of in decrease the chip evacuation torque at deep hole. Reduction in chip evacuation torque was found as not only much higher than that in cutting torque, but also making chance to drill deeper holes. The paper is constructed as follows. Section 2 presents the experiment setup and the plan of the tests. Results and discussion is shown in Section 3. Lastly conclusions and recommendation are given in Section DESIGN OF EXPERIMENTS 2.1. Experimental devices A schematic diagram of the experimental setup is shown in Figure 1. Figure 1 Schema of the experimental tests 1 Rotary vibration actuator, 2 Lathe chuck, 3 Drill bit, 4- Thermal sensor, 5 Workpiece, 6 Fig, 7 - Bearing, 8 Force sensor, 9 Torque sensor, 10 Signal conditioner, 11 DAQ, 12 & 13 Computer, 14 Ultrasonic power generator, 15 Connector, 16 Lathe carriage. Universal lathe machine (V-Turn 410) was used for implementing the experimental tests. Workpieces were made Al6061-T6 in the form of square bars with dimensions of 10x10x30 mm 3. The drilling experiments were carried out using HSS twist drilling bits with diameter of 3 mm and under dry cutting conditions. The drilling torque was measured by a torque sensor model PCB A. The axial force was measured by a force sensor Kistler 9257BA. A computer which utilizes the software NI-Labview Signal Express was used to collect signal data via DAQ model USB A ultrasonic generator model MPI WG-3000 WG was used to convert 50 Hz electrical supply to high-frequency electrical impulses. The frequency range of the generator is 20 to 40 khz and the frequency step is 1 Hz. Figure 2 presents construction of the experimental rotary device. Figure 2 Structure of the ultrasonic vibratory unit with drill bit editor@iaeme.com

4 A Study on the Reduction of Chip Evacuation Torque in Ultrasonic Assisted Drilling Of Small and Deep Holes A commercial ultrasonic welding transducer including a proper horn and booster was fitted into a self-made steel tube and clamped at the node of the horn. The horn was modified by making a hole to put a commercial collet inside. The drilling bit was clamped inside the collet. Steady electrical impulses were supply to the transducer via a copper brush which was positioned at the end of and rotated along with the lathe spindle. Using this arrangement, the drilling bit was simultaneously received both rotation ( the lathe spindle) and longitudinal vibration ( the transducer). In this study, a commercial ultrasonic transducer working with frequency of 25 khz and a stepped horn were selected. The experimental plan was built following a special central composite design, named face-centered plan, using two cutting parameters at three levels each, as given in Table 1. All experiments were repeated three times. Table 1 Cutting parameters and their levels in experiments 3. RESULTS AND DISCUSSION Parameters Level rate The cutting and the chip evacuation force and torque Figure 3 depicts examples of the two components of the force and torque data obtained. Figure 3 Two components of (a) the force and (b) of the torque As can be seen on Fig.3, the thrust force (Figure 3a) and the torque (Figure 3b) keep constant only in the range L/D<2. When the drilling depth increase, both the total force and total torque start increasing. The chip evacuation force (F Ce ) appears to be much larger than the cutting force (F cut ). The chip evacuation torque (T Ce) is also much higher than the cutting torque (T cut ). To make it easy to compare, both force and torque signals were normalized by dividing the signal by the corresponding cutting component. Figure 4 presents the normalized torque and force in CD (Figure 4a) and (Figure 4b). As can be seen the Figure, when the deep increases, both the CD s force and force slightly increase about less than twice compared to the cutting force. However, the torque in CD quickly increases by about 11 times higher than the cutting torque (Figure 4a) and that of 10 times in (Figure 4b). It would be noted that the excessive torque is the main reason for breakage of drill bits editor@iaeme.com

5 Van-Du Nguyen and Ngoc-Hung Chu Figure 4 Normalized of torque and force in (a) a CD test and (b) a test The critical depth is defined as the depth at which an excessive torque signal occurs, resulting in the tool breaking. Data experiments where the drill bits were broken have been collected. Figure 5 presents an example of thrust force and torque during a drill was broken. Figure 5 The force and torque signal when a drill was broken Figure 6 Define the critical depth As can be seen in Fig. 5, the total torque at the instant of the tool breaking was Ncm, as higher as 21.2 times than the cutting torque (15.79 Ncm). However, the breaking force was N, as only 2.9 times higher than the axial cutting force ( N). In addition, before the tool was broken ( L/D>8), the torque signal raised up gradually while the force was still remained. From this practical observation, the critical depth is determined based on values of the torque instead of the axial force. The critical torque data have been collected experiments ranged 330 to 370 Ncm. A safety factor of 5 was chosen in this study, making the allowance threshold torque, T a, of 70 Ncm. The critical depths were then determined as depicted in Fig. 6. The smallest aspect ratio L/D where the torque first reaches the threshold value of 70 Ncm was taken to be the critical depth of the examined operation. All the data obtained in the tests as planned will be analyzed to carry out the effect of editor@iaeme.com

6 A Study on the Reduction of Chip Evacuation Torque in Ultrasonic Assisted Drilling Of Small and Deep Holes 3.2. Reduction of cutting torque by mean of The objective of the experiments is to scrutinize the effect of vibration assistance on the cutting torque. Comparison of the cutting torque is made by mean of paired-t test technique. After that, a response surface analysis was made to represent the cutting torque in CD and in by mean of mathematical functions of the cutting parameters. The cutting torque taken experiments for Conventional Drilling (CD) and for Ultrasonic Assisted Drilling (), as shown in Table 2, were used to implement the comparative study. # Table 2 Cutting torque CD and experiments T cut in CD (Ncm) Tcut in (Ncm) # T cut in CD (Ncm) Tcut in (Ncm) The paired t-test is useful for analyzing the same set of data that were obtained under two different conditions, differences between two treatments given to the same subject. In this study, the cutting torque of each hole machined in two different conditions (CD versus ) will be compared in-pair, i.e. in the same cutting parameters. The paired t-test was implemented to determine whether the cutting torque in CD is higher than that in. Using paired-t test, a limited experimental samples, we can predict the difference for the whole population of data. The comparative test was made with a hypothesis declared as: population mean of the cutting torque in CD is higher than that in. This hypothesis can be expressed as: Null Hypothesis H 0 : µ d = 0 Alternative Hypothesis H1 : µ d 0 1 where μ d is the population mean of differences between CDs and s cutting torques. Table 3 presents the analyzed results obtained. Table 3 Results of Paired T-Test obtained Minitab Paired T for TCut_CD - TCut_ N Mean StDev SEMean TCut_CD TCut_ Difference % CI for mean difference: (3.179, 4.494) T-Test of mean difference = 0 (vs not = 0): T-Value = P- Value = editor@iaeme.com

7 Van-Du Nguyen and Ngoc-Hung Chu The confidence interval (95% CI) for the mean difference between the two sets (CDs vs s torques) did not include zero, which suggests a difference between them. The small p- value (p = 0.000) further suggests that the data are inconsistent with H0: µ d = 0, i.e. the two sets of data are not equal. Specifically, CDs cutting torque (mean = ) is higher than cutting torque (mean = ) for all of similar cutting conditions. The mean difference is of 3.837, about 25% of the mean of the CDs cutting torque. In other words, applying can reduce the cutting torque an average of 25%. The data presented in Table 2 were then analyzed by mean of the response surface technique. Figure 7 shows the contour plots of the cutting torque as a function of the speed and feeding rate for CD (Fig. 7a) and for (Fig. 7b). Figure 7 Contour plots of cutting torque in (a) CD tests and (b) tests As can be seen in Fig. 7a, the cutting torque in CD is almost independent cutting speed and increase linearly with the feeding rate. Higher feeding rate results in higher cutting torque. This observation agreed with well-known dependence of the cutting torque on feeding rate, as shown in common manufacturing text books and handbooks. However, as shown in Fig. 7b, the cutting torque depends both on cutting speed and feeding rate. This means that it is possible to drill with ultrasonic assistance at high speed and feed rate while the cutting torque is as low as the cutting torque in conventional drilling under a low feed rate. Nevertheless, this benefit is not practically valuable, since the reduction in cutting torque is not significant (around 3 Ncm), as shown above. The most important aspect of compared to CD is the reduction of the chip evacuation torque at higher depths, as shown in the next subsection Effect of on the critical depth Since the chip evacuation torque is dependent on the drilled depth, the reduction of the chip evacuation torque will be scrutinized by mean of the critical depth, as proposed before. Table 4 presents the data of aspect ratio obtained in CDs and s tests. # Table 4 Critical depth () obtained CD and experiments CD # CD editor@iaeme.com

8 # A Study on the Reduction of Chip Evacuation Torque in Ultrasonic Assisted Drilling Of Small and Deep Holes CD # CD The data presented in Table 4 were then analyzed by mean of the response surface technique. Figure 8 shows the contour plots of the critical depth, as a function of the speed and feeding rate for CD (Fig. 8a) and for (Fig. 8b). Figure 8 Contour plots of critical depth in (a) CD tests and (b) tests In overall, the critical depth obtained CD tests ranged 5.2 to 6.8 times of the hole diameter, while that s ranged 6 to 9.5 times of the hole diameter. In other words, the depth can be safety reached is as higher as 1.4 times than that CD. The maximum depth can be reached in CD is of 6.8, appeared at low speed (around 1000 rpm) while such depth can be reached at 1500 rpm and feed rate of This means that with the same required depth, can be performed under higher speed and feed rate, resulting in higher production rate. Also, as shown in Fig 8b, with the feed rate 0.75 to 0.85 mm/rev and the cutting speed 1000 to 1100 rpm, the hole depth which can be safety drilled is as deep as 9.5 and higher times of the hole diameter. This is the most important advantage of compared to CD. 4. CONCLUSION This paper presented an experimental study to examine the advantage of ultrasonic assisted drilling (), focusing on the ability of to safety lengthens the critical depth in small and deep drilling. The experimental plan and the results have been implemented using the theory of experiment design and statistical technique, by mean of the Minitab software. It has been shown that, ultrasonic assisted drilling can reduce the cutting torque by 25% compared to conventional drilling. Moreover, it has been found that provided a much higher range of the hole depth which can be safety drilled. The reductions of cutting torque by have been found in several previous studies. However, the reduction of the chip evacuation torque, resulting in keeping the total torque lower and thus making can produce deeper holes would not have been analyzed yet. The ultrasonic assistance can be also combined with grooving technique and/or peck drilling to further develop in deep drilling technique editor@iaeme.com

9 Van-Du Nguyen and Ngoc-Hung Chu ACKNOWLEDGEMENT The authors would like to express their thanks to supports for the study at initial stages, the Grants number B2013-TN02-02 and DH2013-TN Conflict of interest: The authors declare that there have no conflicts of interest. Ethical statement: The authors declare that we have followed ethical responsibilities. REFERENCES [1] Liu, H.S., B.Y. Lee, and Y.S. Tarng, In-process prediction of corner wear in drilling operations. Journal of Materials Processing Technology, (1): p [2] Arzur-Bomont, A., et al., Machinability in drilling mechanistic approach and new observer development. International Journal of Material Forming, (S1): p [3] Ke, F., J. Ni, and D.A. Stephenson, Chip thickening in deep-hole drilling. International Journal of Machine Tools and Manufacture, (12-13): p [4] Nath, C. and T. Kurfess, Obstruction-type Chip Breakers for Controllable Chips and Improved Cooling/Lubrication during Drilling A Feasibility Study. Procedia Manufacturing, : p [5] Rivero, A., et al., An experimental investigation of the effect of coatings and cutting parameters on the dry drilling performance of aluminium alloys. The International Journal of Advanced Manufacturing Technology, (1-2): p [6] Zheng, H. and K. Liu, Machinability of Engineering Materials. 2015: p [7] Ashrafi, S.A., et al., Investigation into Effect of Tool Wear on Drilling Force and Surface Finish While Dry Drilling Aluminum Advanced Materials Research, : p [8] Roy, P., et al., Machinability study of pure aluminium and Al 12% Si alloys against uncoated and coated carbide inserts. International Journal of Refractory Metals and Hard Materials, (3): p [9] Drozda, T.J., Tool and Manufacturing Engineers Handbook: Machining. 1983: Society of Manufacturing Engineers. [10] Mellinger, J.C., et al., Modeling Chip-Evacuation Forces and Prediction of Chip-Clogging in Drilling. Journal of Manufacturing Science and Engineering, (3): p [11] Mellinger, J.C., et al., Modeling Chip-Evacuation Forces in Drilling for Various Flute Geometries. Journal of Manufacturing Science and Engineering, (3): p [12] Nagao, T., Y. Hatamura, and M. Mitsuishi, In-Process Prediction and Prevention of the Breakage of Small Diameter Drills Based on Theoretical Analysis. CIRP Annals, (1): p [13] Furness, R.J., A.G. Ulsoy, and C.L. Wu,,, and Torque Controllers for Drilling. Journal of Engineering for Industry, (1): p. 2. [14] Tom Mathew, N. and L. Vijayaraghavan, Dry Deep Drilling of Titanium Aluminide. ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE) Volume 2A-2015, 2015: p. V02AT02A038. [15] Sahu, S.K.B.O., O. DeVor, Richard E. Kapoor, Shiv G., Effect of groove-type chip breakers on twist drill performance. International Journal of Machine Tools and Manufacture, (6): p [16] Ravisubramanian, S. and M.S. Shunmugam, Investigations into peck drilling process for large aspect ratio microholes in aluminum 6061-T6. Materials and Manufacturing Processes, (9): p editor@iaeme.com

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